The present invention relates generally to wafer probe tests. More particularly, an improved method of determining the contact resistance between a probe card and a device under test during a wafer probe test is disclosed.
Integrated circuits (ICs) are typically fabricated by groups on wafers. After the fabrication has been completed, each of the ICs must be tested to insure that it works properly. This is typically accomplished using a test head that includes a probe card. The probe card has a multiplicity of probe contacts that are designed to engage the pins of a selected one of the integrated circuits (ICs). During testing, the probe is positioned over a wafer such that the probe contacts contact the pins of a selected chip on the wafer. A test program is then executed to determine whether the chip performs satisfactorily.
One problem frequently encountered during testing is the failure to make a good contact between the probe contacts and the IC pins. If a good contact is not made, there is a risk that a perceived failure will be the result of a poor contact rather than due to an actual defect in the chip. In single pass testing of state of the art ICs, using conventional testing equipment and conventional testing techniques, it is not uncommon to have chips fail due to poor contacts.
In order to address this problem, it is common to at least periodically attempt to measure the contact resistance during testing. If such tests reveal that the contact is getting bad, appropriate measures are taken to remedy the problem. By analyzing the contact resistance tests results, engineers can develop guidelines as to how often the probers should be cleaned during testing, and can see the development of other problems such as the buildup of contamination on the probe contacts.
The most common method of calculating the contact resistance is to force a designated current through the diode that is typically present between the device pin and the ground (or power supply). The voltage drop associated with the forced current is then measured. If a bad contact exists, the voltage drop will increase. In practice, such measurements measure the resistance of two contacts in addition to the internal resistance of the device. In cases where the contact is checked by forcing current between the device pin and ground, the internal resistance may take the form of the resistance of the internal supply lines and the resistance of the diode. In such a circuit, the internal resistance may be on the order of 5-1000 ohms and the contact resistance of a good pair of connections may be on the order of 0.1-5 ohms. On the other hand, the contact resistance of a pair of bad connection may be on the order of 5-50 or more ohms. A typical forced current is on the order of 0.1-10 milliamps and up. Thus, the expected voltage drop may be on the order of 0.5-1.5 volts. Thus, the total resistance measured may be on the order of 50 ohms and up, while the differential resistance to be measured (that is the variations in contact resistance) may be on the order of 1 to 100 ohms and up (depending on the device technology etc.). Of course, these numbers may vary a great deal in accordance with the devices being tested. Accordingly, it is difficult to analyze the "contact resistance data" since the resolution is necessarily poor.